Default id to being a read only (not creatable) attribute

Fixes #960

Potentially a breaking change for apps with guids

(cherry picked from commit 4f51331)

Author: lgebhardt

lib/jsonapi/resource.rb

@@ -430,7 +430,9 @@ def inherited(subclass)
         type = subclass.name.demodulize.sub(/Resource$/, '').underscore
         subclass._type = type.pluralize.to_sym
 
-        subclass.attribute :id, format: :id
+        unless subclass._attributes[:id]
+          subclass.attribute :id, format: :id
+        end
 
         check_reserved_resource_name(subclass._type, subclass.name)
       end
@@ -598,7 +600,7 @@ def updatable_fields(_context = nil)
 
       # Override in your resource to filter the creatable keys
       def creatable_fields(_context = nil)
-        _updatable_relationships | _attributes.keys
+        _updatable_relationships | _attributes.keys - [:id]
       end
 
       # Override in your resource to filter the sortable keys

test/fixtures/active_record.rb

@@ -1120,7 +1120,7 @@ def self.updatable_fields(context)
   end
 
   def self.creatable_fields(context)
-    super(context) - [:subject, :id]
+    super(context) - [:subject]
   end
 
   def self.sortable_fields(context)
@@ -1142,6 +1142,10 @@ class HairCutResource < JSONAPI::Resource
 class IsoCurrencyResource < JSONAPI::Resource
   attributes :name, :country_name, :minor_unit
 
+  def self.creatable_fields(_context = nil)
+    super + [:id]
+  end
+
   filter :country_name
 
   key_type :string